Ultrasonic echo-ranging has gained wide application in both liquid and solid particulate level sensing. Such systems typically provide a transducer positioned above the material whose level is to be measured. An ultrasonic burst at, for example, 50 kHz, is transmitted downwardly towards the material surface and reflected therefrom. The echo is received by the transducer and detected; that is, discrimination may be performed. The round-trip transit time between the transmission of the burst and the reception of the echo is directly proportional to the distance from the transducer to the material surface.
Ultrasonic ranging systems conventionally employ piezoelectric type transducers, often with a single element serving as both transmitter and receiver. A high frequency electrical pulse applied to a piezoelectric crystal causes it to vibrate, emitting an acoustic pulse; if a pressure variation is applied to the crystal, a voltage is produced. The transducer thus converts acoustic to electrical energy, and vice versa. Unfortunately, the mechanical inertia of the crystal significantly limits the performance in pulsed applications. The response is slow, and "ringing" of the crystal after transmission of the burst limits the minimum distance that can be measured.
Electrostatic "Sell type" transducers which comprise a highly flexible, partially metallic diaphragm spaced from an electrode, offer several advantages over the more conventional piezoelectric devices, while operating is an essentially similar fashion. The mass of the air moving element, the diaphragm, may be made extremely low, permitting both fast response and minimum ringing.
Such transducers, though, pose several unfamiliar interfacing problems. The element must be driven with on the order of several hundred volts for reasonable transmit energy, while an extremely well filtered dc bias must be applied to operate in the receive mode. Further, the efficiency is generally less than with conventional piezoelectric elements, thereby requiring high receiver gain, and putting additional burdens on the noise discrimination circuitry and transmission line.
It is often desirable to multiplex several transducers in a level measurement system, so that a single processing circuit may be used to, e.g., monitor the level in many tanks. With conventional piezoelectric transducers, often interfaced to with balanced lines for noise rejection, multiplexing poses several difficulties. Low signal levels, balanced lines, capacitance effects, noise and cross-talk must be carefully considered and compensated for. Often, due to the aforementioned problems, the cable lengths and number of transducers must be limited, thus diminishing the utility of such a system. Nor are such problems overcome by the direct substitution of electrostatic for piezoelectric transducers; indeed, the low signal levels and high voltage requirements of electrostatic transducers in many cases increases these difficulties.